Synthesis of purine LNA (Locked Nucleic Acid) monomers were first reported by Wengel et al. (Singh, S. K.; Nielsen, P., Koshkin, A. A. and Wengel, J., Chem. Commun., 1998, 455; Koshkin, A. A.; Singh, S. K.; Nielsen, P.; Rajwanshi, V. K.; Kumar, R.; Melgaard, M.; Olsen, C. E. and Wengel, J., Tetrahedron, 1998, 54, 3607). Using a convergent strategy the two purine LNA monomers (1S,3R,4R,7S)-7-hydroxy-1-hydroxymethyl-(2-N-isobutyrylguanine-9-yl and 6-N-benzoyladenin-9-yl)-2,5-dioxabicyclo[2.2.1]heptane were synthesized in 5 steps from the key intermediate, 4-C-acetoxymethyl-1,2-di-O-acetyl-3,5-di-O-benzyl-D-ribofuranose. Glycosylation of the key intermediate with silylated 2-N-isobutyrylguanine and 6-N-benzoyladenin afforded the 4′-C-acetoxymethyl guanine and adenine nucleosides, respectively, which upon deacetylation and monotosylation followed by base-induced ring closure, afforded the corresponding 2′-O,4′-C-methylene bicyclonucleosides. Final debenzylation afforded (1S,3R,4R,7S)-7-hydroxy-1-hydroxymethyl-(2-N-isobutyrylguanine-9-yl and 6-N-benzoyladenin-9-yl)-2,5-dioxabicyclo[2.2.1]heptane in 16% and 14% overall yields (calculated from the key intermediate), respectively. The low overall yields are primarily due to the glycosylation reactions which afford isomeric mixtures of the nucleoside analogues in ≈50% yields and the subsequent intramolecular ring closure of the C-branched carbohydrate which processed in less than 44% yield. Analogous synthetic procedures were applied for the synthesis of the thymine, uracil and 4-N-benzoylcytosine LNA nucleoside analogues.
An analogous convergent synthesis of (1S,3R,4R,7S)-7-hydroxy-1-hydroxymethyl-3-(2-N-isobutyrylguanin-9-yl and adenin-9-yl)-2,5-dioxabicyclo[2.2.1]heptane has been reported by Pfundheller, H. M. et al (Lomholt, C., Koshkin, A. A., Fensholdt, J., Meldgaard, M., and Pfundheller, H. M., poster presented at XIV International Roundtable; Nucleoside and Nucleotides and Their Biological Applications, Sep. 10-14, 2000; San Francisco, USA). Starting from the key intermediate 1,2-di-O-acetyl-3-O-benzyl-4-C-methanesulfonylmethyl-5-O-methanesulfonyl-D-ribofuranose the purine LNA monomers were obtained in 50% overall yields. This strategy, however, still suffer from the drawback of formation of isomeric mixtures in the glycosylation reaction requiring separation by chromatographic methods.
Characteristic properties of the previously known strategies discussed above are relatively low overall yields, many synthetic steps and the need of chromatographic methods for the separation of isomeric mixtures. Thus, there is a great need for development of a more efficient synthesis strategy for purine LNA analogues that will result in an improvement of the overall yield and a reduction in the production costs.